Ok, this is one of the weirdist problems I've ever had with a game.  Let me describe it:

I play the game (great fun, loving it btw) just fine with no issues in gameplay.

BUT, there is a weird buzzing noise coming from my PC when I play.

No, I am not a stupid moron who hasn't noticed a furred up fan or a loose power cable.  This noise ONLY happens when I am playing SoaSE. 

At first, I thought it was probably the fan on my graphics card or CPU ramping up to cope with increased temperatures from playing the game.  But having opened up the case and listend carefully, I'm pretty sure the buzzing noise is coming from the PSU.  Also, the very INSTANT i stop playing, or even task out, the noise vanishes, which would be inconsistent with a fan-spin noise, which you'd expect to take a couple of seconds at least to go down.

The noise is very irritating.  But what is very, very strange is the fact that it only occurs when I am playing this game.  It has never happened before.  I do not get this noise when playing Crysis, TF2, World in Conflict, or Medieval TW2.  Only Sins causes it.

In fact, I have just tasked out of Sins to write this post, and the noise stops instantaneously when I task out.  But when I go back in (*goes back into game to check*), it starts up again at once.

I wondered if it might be the long-forgotten remnant of the PC Speaker doing something funny - like trying to play the Sins soundtrack by itself, and interpreting it as a constant buzz?  But this is just a wild guess, and I have no idea how that could happen or how I could fix it - and anyway, the sound really does seem to be coming out of the PSU.

I have very little hope that anyone will be able to solve this peculiar problem, but any thoughts would be much appreciated.